Field Effect Transistors (FETs) are well known to be ideal for use in applications requiring amplification or switching at radio or microwave frequencies. FETs fabricated primarily of GaAs are particularly suited for high frequency uses because of the high electron mobility characteristic of this compound semiconductor. In the past, FETs have utilized a Shottky barrier gate structure (hence the common name Metal-semiconductor Field Effect Transistor, or MESFET) and have been fabricated on semi-insulating GaAs substrates with all dopants being ion implanted.
Recently, the performance demands of modern radar and telecommunications equipment have outstripped the capabilities of traditional MESFET technology. Consequently, FETs have evolved into largely epitaxial structures where semiconductor layers are precisely grown and doped in situ in the growth process. This has allowed the use of highly doped, precisely defined channel regions buried beneath lightly doped buffer layers, which in turn results in "low-high" FETs with a highly linear relationship between transconductance and gate voltage, a characteristic that is important in minimizing distortion in a high-frequency transistor amplifier.
Additionally, FETs have evolved to permit operation with higher breakdown voltages and therefore at higher power than was possible with traditional MESFETs. One method to achieve higher breakdown voltages in the past has been to incorporate an AlGaAs buffer layer atop a GaAs channel layer. The AlGaAs layer is undoped or lightly doped and separates the highly doped GaAs channel from the gate contact placed on top of the AlGaAs layer. This device is known generally as a MISFET (Metal Insulator Field Effect Transistor) because of the "insulating" AlGaAs layer.
Radar and telecommunications systems commonly require the low-noise highly linear performance of "low-high" FETs for receiver amplifiers, while also requiring the high power, robust structure characteristic of a MISFET type transistor amplifier for transmitting applications. This has traditionally required a system designer to have integrated circuits for power amplifiers, integrated circuits for low-noise amplifiers, and even integrated circuits for the switching and phase shifting functions commonly used in these systems.